(1) What is the maximum length of a subdomain? 子域名的最大长度是多少？
63
(2) Which of the following characters cannot be used in a subdomain ( 3 b _ - )? 以下哪个字符不能在子域（3b_-）中使用？
_
(3) What is the maximum length of a domain name? 域名的最大长度是多少？
253
(4) What type of TLD is .co.uk? .co.uk是什么类型的顶级域名？
ccTLD